1. Technical Field
This invention relates to free-space laser communication systems, and more particularly to a method and apparatus for controlling the dynamic range of a received laser beam in a free-space laser communications system.
2. Background Information
Free-space laser communication systems transmit and receive information by means of a light beam that propagates through space or the atmosphere. When used for air-to-air or air-to-ground communications, such systems pose a number of challenging problems.
One such problem is high variability in the laser beam signal level. Air-to-air tracking applications using laser xe2x80x9cbeaconsxe2x80x9d may involve enormous changes in signal level. Typically, tracking an optical beacon of constant output over ranges from a few kilometers to hundreds of kilometers means detecting and processing signals whose level may change by four orders of magnitude or more. For example, tracking a 110 mW laser beacon from 5 km to 500 km means a signal that changes by a factor of 10,000. Thus, in engineering such systems, designers need to plan for about 80 dB of dynamic range.
Most xe2x80x9cwidexe2x80x9d field of view, high resolution laser communication tracking systems employ a charge coupled device (CCD) array. CCD""s generally have excellent sensitivity and noise performance, can be clocked at high speed (for high bandwidth applications), and are easily integrated into tracking control loops. However, current CCD""s do not have sufficient dynamic range to accommodate the great variability of signal levels in laser communication systems. Other imaging devices, such as photoconductor-on-CMOS devices, have similar problems.
Several approaches to solve this problem generally fall short of adequate solutions. For example, a variable iris for controlling the dynamic range of the incoming laser beam signal would need to be placed in the optical train at a point where it would not impact the system field of view. In addition, such an iris must be precisely aligned on the optical axis, since any asymmetric vignetting of the laser light would result in centroid (and thus pointing) errors. Lastly, an iris must be capable of providing at least 20 dB of attenuation from its position in the optical train. Ultimately the physical tolerances (on the order of a few microns), alignment difficulties and extinction requirements for a variable iris make this option undesirable.
As another example, neutral density (ND) filters or circular variable filters (CVF""s) could be mechanically inserted in the optical train. ND filters would not impact the field of view, but optics inserted into the beam train would have to be carefully placed to prevent beam steering. Such beam steering would again result in significant centroid and pointing errors. CVF""s could be placed in the beam train permanently, but variable thickness (xe2x80x9cwedgexe2x80x9d) in the filter itself would cause beam steering as the filter was rotated.
As yet another example, liquid crystal shutters could be inserted in the optical train.
Although these devices suffer from none of the limitations of irises or filters, they are very limited in dynamic range, with extinction ratios typically of only about 4-5. This is far below the necessary extinction ratio of 100 or greater.
The invention includes a liquid crystal variable retarder (LCVR) with automatic gain control for use with an imager-based target tracking application such as a free-space laser communication system. More particularly, the LCVR is part of a feedback system that adjusts the intensity of the incoming laser signal on a timescale appropriate to the tracking problem at hand. For an air-to-air free-space laser communication system, this signal compensation happens on the order of seconds to minutes.
A retarder (or waveplate) is an optical device that resolves a light wave into two orthogonal linear polarization components and produces a phase shift between the components. The resulting light wave is generally of a different polarization form. Ideally, retarders do not polarize, nor do they induce an intensity change in the light beam, but simply change the polarization form of the light beam. An LCVR is made of two optical windows separated by a gap, typically of a few microns. The gap is filled with nematic liquid crystal material. Electrodes are situated to enable an electric field to be applied between the optical windows and thus across the liquid crystal material. With no voltage applied to the electrodes the liquid crystals are co-aligned and lie parallel to the optical windows. In this state of operation, the LCVR exhibits maximum retardation. As voltage is applied to the electrodes, the co-aligned liquid crystal molecules rotate away from the optical windows, becoming perpendicular to the optical windows. In this state of operation, the LCVR exhibits minimum retardation.
A preferred embodiment of the invention includes an optical train having a first polarizer, a narrow-bandpass filter, a second polarizer orthogonal to the first polarizer, a liquid crystal variable retarder, and a third polarizer orthogonal to the second polarizer. The light then passes through focusing optics that image the light onto an imaging array, such as a CCD device, or a photodetector. A feedback circuit controls the liquid crystal variable retarder to provide variable attenuation of an incoming light beam.
An LCVR avoids the limitations of irises, neutral density filters, and LCD shutters. An LCVR can be placed permanently in the beam train of a receiver and effects of the LCVR on the incoming laser beam can be precisely calibrated. LCVR""s contain no moving parts and thus will not contribute to any beam steering. The transmission of an LCVR in the band of interest typically can be better than 95%. The level of extinction of an LCVR depends on the quality of external polarizers, which can typically provide extinction ratios of 100 to 1000.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.